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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

K-shell photoionization of multielectron atomic systems

A simplified procedure is proposed for calculating the cross section for photoionization from the K-shell for a general atomic system that contains an arbitrary number of outer-shell electrons. The procedure retains the formalism of the one-electron atom case (pure Coulomb-field problem) by determining the effective nuclear charge reduced from Z by 'screening parameters' associated with the other K-shell electron and the outer-bound electrons. The parameters are determined essentially by fitting the atom's radial electric field to Coulomb form at the distance from the nucleus where the integrand of the dipole radial integral is peaked.

Gould, R. J.↗
NASA NTRS1980

Cold dense photoionized universes

This investigation employs a computer code, initially constructed for modeling the evolution of a hot dense intergalactic medium (IGM), to study some properties of a cold dense IGM photoionized by QSOs. Within the observational constraints set by flux measurements in the far-ultraviolet, night sky brightness, and hard X-ray region, and the optical depth limits on various QSO spectra, it determines some allowable scenarios for a range of QSO spectral indices (alpha) and cutoff energies. With H(0) = 50 km/s-Mpc, closure density of an IGM (composed of H and He in their cosmic ratio) is possible only if alpha is greater than or equal to 0.1, a circumstance not likely to prevail; the most probable value, alpha = -0.7, implies an upper limit density about 0.35 of closure, which is somewhat larger than previous estimates. These estimates are substantially independent of whether or not QSOs produce the observed diffuse extragalactic X-ray background; such QSO spectral details are critical, however, in determining the ionization of heavy elements that are expected to contaminate a pure primeval plasma.

Sherman, R. D.↗
NASA NTRS1981

The production rate of C/+/ from the photoionization of CO and CO2

The dissociative photoionization cross sections for the production of C(+) ions are presented for the spectral region from the dissociative ionization threshold to 90 A. The cross sections are considerably larger than previously published data and should therefore influence the calculations of the C(+) concentrations in the Mars and Venus ionospheres. It is suggested that the released energy in the C(+) fragment ion could reduce the loss rate of the ions in collisional processes, thereby accounting for the apparent discrepancies between the observed and calculated C(+) ion density in the Venus ionosphere.

Samson, J. A. R.↗
NASA NTRS1983

Modification of the photoionization of hydrogen by a low-frequency laser

The modification of the photoionization cross section due to the presence of an intense low-frequency laser is obtained for the hydrogen-atom target. Previous calculations of this effect have neglected the effects of the Coulomb potential in the final state. A sum rule was obtained which indicates that the photoelectric current is unaffected by the presence of the low-frequency laser to lowest order in the laser frequency. It is found that the Coulomb potential changes this result and that the photoelectric current is changed by a factor which depends upon the laser parameters and the electron momentum. A simple experiment is suggested to confirm this prediction.

Fiordilino, E.↗
NASA NTRS1983

Photoionization models for the winds from cataclysmic variables

Photoionization models are presented for the relative abundances of Si IV, C IV, and N V as a function of the ionization parameter and shape of the ionizing continuum spectrum in the EUV and X-ray energy range. These models are applied to the interpretation of the UV resonance lines observed from the cataclysmic variable TW Virginis by Cordova and Mason. The observed lines are assumed to be formed in gas flowing supersonically out from the continuum source in the form of a stellar wind. It is shown that the shape of the ionizing spectrum can be determined without assuming that the outflow is spherical by fitting to ratios of the line strengths. Models which fit the observed ratios include an X-ray continuum with a low-energy cutoff and may also include a lower luminosity component in the EUV spectral region. Under the added assumption of spherical outflow the derived ionization parameter for the X-ray component can be used to constrain the total mass flux in the wind.

Kallman, T. R.↗
NASA NTRS1982

Absolute photoionization cross sections of atomic oxygen

The absolute values of photoionization cross sections of atomic oxygen were measured from the ionization threshold to 120 A. An auto-ionizing resonance belonging to the 2S2P4(4P)3P(3Do, 3So) transition was observed at 479.43 A and another line at 389.97 A. The experimental data is in excellent agreement with rigorous close-coupling calculations that include electron correlations in both the initial and final states.

Samson, J. A. R.↗
NASA NTRS1985

Kinetic energies of fragment ions produced by dissociative photoionization of NO

The kinetic energies of ions produced by dissociative photoionization of NO have been measured at the discrete resonance lines of He (584A) and Ne (736A), and with undispersed synchrotron radiation. O sup + ions were identified with energies from 0 to approximately 0.5 eV and two groups of N sup + ions one with energy of 0.36 eV and another with energies between 0.9 and 1.5 eV, apparently produced by predissociation of the C sup 3 P 1 and B'1 sigma states respectively.

Samson, J. A. R.↗
NASA NTRS1984

Soft X-ray opacity in hot and photoionized gases

New calculations are presented of the soft X-ray opacity of gas having cosmic elemental abundances, but in a variety of ionization states. Coronal ionization equilibrium is considered over the range of temperatures 10 to the 4th to 3 x 10 to the 8th K and photoionization equilibrium with power-law and thermal bremsstrahlung spectra over a wide range of ionization parameters. The results are displayed in illustrative figures. For temperatures and ionization parameters commonly found in astrophysical plasmas, the opacity (particularly to photons of equal to or less than 1 keV) can be diminished by substantial factors relative to the state in which all atoms are neutral. As the dominant ionization stages change, the positions and contrasts of prominent K-edges also change.

Krolik, J. H.↗
NASA NTRS1985

Absolute photoionization cross sections of atomic oxygen

The absolute values of photoionization cross sections of atomic oxygen were measured from the ionization threshold to 120 A. An auto-ionizing resonance belonging to the 2S2P4(4P)3P(3Do, 3So) transition was observed at 479.43 A and another line at 389.97 A. The experimental data is in excellent agreement with rigorous close-coupling calculations that include electron correlations in both the initial and final states.

Samson, J. A. R.↗
NASA NTRS1985

Photoionization models for the wind from TW Vir

Line profiles are examined for the resonance doublets of the ions C IV, Si IV, and N V in the spectrum of the dwarf nova TW Virginis as observed by Cordova and Mason (1982). Line strengths depend on the abundances of the scattering ions and hence on the physical characteristics in the stellar wind. Results of the study show that the observed UV line ratios from TW Vir cannot be produced by photoionization by either an unattenuated single bremsstrahlung or by a single blackbody spectrum. Satisfactory values of the abundance ratios can be produced by bremsstrahlung spectra extending into the X-ray region which have low-energy cutoffs near the N IV threshold energy at 77 eV and have fluxes corresponding to an ionization parameter xi-sub-br of about 10. A finding that the flux in the soft X-ray bremsstrahlung component must exceed the EUV blackbody flux by at least an order of magnitude has implications for theories of the continuum emission from cataclysmic variables.

Kallman, T.↗
NASA NTRS1985

Kinetic energies of fragment ions produced by dissociative photoionization of NO

The kinetic energies of ions produced by dissociative photoionization of NO have been measured at the discrete resonance lines of He (584A) and Ne (736A), and with undispersed synchrotron radiation. O sup + ions were identified with energies from 0 to approximately 0.5 eV and two groups of N sup + ions one with energy of 0.36 eV and another with energies between 0.9 and 1.5 eV, apparently produced by predissociation of the C sup 3 P 1 and B'1 sigma states respectively.

Samson, J. A. R.↗
NASA NTRS1986

Total absorption and photoionization cross sections of water vapor between 100 and 1000 A

Absolute photoabsorption and photoionization cross sections of water vapor are reported at a large number of discrete wavelengths between 100 and 1000 A with an estimate error of + or - 3 percent in regions free from any discrete structure. The double ionization chamber technique utilized is described. Recent calculations are shown to be in reasonable agreement with the present data.

Haddad, G. N.↗
NASA NTRS1986

Total and dissociative photoionization cross sections of N2 from threshold to 107 eV

The absolute cross sections for the production of N(+) and N2(+) were measured from the dissociative ionization threshold of 115 A. In addition, the absolute photoabsorption and photoionization cross sections were tabulated between 114 and 796 A. The ionization efficiencies were also given at several discrete wave lengths between 660 and 790 A. The production of N(+) fragment ions are discussed in terms of the doubly excited N2(+) states with binding energies in the range of 24 to 44 eV.

Samson, James A. R.↗
NASA NTRS1987

Total and dissociative photoionization cross sections of N2 from threshold to 107 eV

The absolute cross sections for the production of N(+) and N2(+) have been measured from the dissociative ionization threshold to 115 A. In addition, the absolute photoabsorption and photoionization cross sections are tabulated between 114 and 796 A. The ionization efficiencies are also given at several discrete wavelengths between 660 and 790 A. The production of N(+) fragment ions are discussed in terms of the doubly excited N2(+) states with binding energies in the range 24 to 44 eV.

Samson, James A. R.↗
NASA NTRS1987

Total photoionization cross sections of atomic oxygen from threshold to 44.3A

The relative cross section of atomic oxygen for the production of singly charged ions has been remeasured in more detail and extended to cover the wavelength range 44.3 to 910.5 A by the use of synchrotron radiation. In addition, the contribution of multiple ionization to the cross sections has been measured allowing total photoionization cross sections to be obtained below 250 A. The results have been made absolute by normalization to previously measured data. The use of synchrotron radiation has enabled measurements of the continuum cross section to be made between the numerous autoionizing resonances that occur near the ionization thresholds. This in turn has allowed a more critical comparison of the various theoretical estimates of the cross section to be made. The series of autoionizing resonances leading to the 4-P state of the oxygen ion have been observed for the first time in an ionization type experiment and their positions compared with both theory and previous photographic recordings.

Angel, G. C.↗
NASA NTRS1989

On star formation in stellar systems. II - Photoionization in protodwarf galaxies

Numerical hydrodynamical calculations are used to study the effects of the onset of star formation on the residual gas in a primordial low-mass Local-Group dwarf spheroidal galaxy in the size range 0.3-1.0 kpc. It is demonstrated that photoionization in the presence of a moderate gas-density gradient can be responsible for gas ejection on a time-scale of a few times 10 to the 7th yr. The results indicate that, given a normal initial mass function, many protodwarf galaxies may have been dispersed by the onset of star formation.

Noriega-Crespo, A.↗